JP2816097B2 - Rare earth element-doped multi-core optical fiber, method for manufacturing the same, and optical amplifier using the optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth element-doped multi-core optical fiber in which a plurality of cores containing at least a rare earth element such as Er and Yb and Al are provided in a cladding having a low refractive index. The present invention relates to a multi-core optical fiber structure realizing characteristics, a manufacturing method thereof, and an optical amplifier using the same.

[0002]

2. Description of the Related Art In recent years, Er, N
By adding a rare-earth element such as d or Pr (hereinafter, this core is referred to as a rare-earth-doped single-core fiber) and exciting excitation light having an absorption characteristic inherent to the rare-earth element added to the optical fiber, a signal is obtained. Research and development of optical fiber amplifiers for amplifying light have been activated. Among them, Er
There is a remarkable improvement in the performance of an optical fiber amplifier to which is added, and at present, a high gain of 40 dB or more is realized.

As shown in FIG. 13, the present inventor has proposed a rare-earth-doped multi-core optical fiber having a multi-core having a plurality of high-refractive-index cores 2 containing a rare-earth element in a low-refractive-index cladding 3. No. 1 has been developed for the first time in the world, and it has already been proposed that a high-gain power amplifier can be realized (JP-A-5-299733, JP-A-6-37385).
issue). With the realization of such a high-gain optical amplifier, transmission of wavelength-multiplexed optical signals of ten or more channels has been studied. In this case, what is required for the optical fiber amplifier is a so-called broadband optical fiber amplifier that amplifies signal light uniformly over a wide wavelength band. Various attempts have been made to develop such a broadband optical fiber amplifier.

[0004] The first attempt is a method of widening the bandwidth at the expense of gain because the band characteristic becomes very narrow in order to obtain the maximum gain. In this method, rather than setting the signal light input to a small signal level, the signal light input is set to a considerably large medium signal input. The second attempt is to increase the bandwidth by adding an extremely high concentration (〜3%) of Al into the core of the optical fiber.

A third attempt is a method of sacrificing the gain similarly, but is a method of widening the band by using an optical fiber that is sufficiently shorter than the optimum length of the optical fiber for optical amplification. The fourth attempt is to increase the bandwidth by connecting optical fiber amplifiers in multiple stages and inserting a variable attenuator between the optical fiber amplifiers to suppress the gain.

A fifth approach is to insert a waveguide type Mach-Zehnder type optical filter between multi-stage optical fiber amplifiers to achieve a wider band by equalizing gain in a desired wavelength band. A sixth attempt is to make the gain band characteristics of the optical fiber amplifier uniform by controlling the power of each wavelength of the wavelength multiplexing signal light source.

[0007]

However, an optical amplifier using a conventional multi-core optical fiber has the following problems. (1) Since the first attempt sacrifices the gain, it can be used only under special conditions in which the powers of the signal light and the pump light are restricted. (2) The second attempt is to add Er in the core of the optical fiber amplifier.
In addition, the addition of Al is excellent in widening the band by adding Al. However, the upper limit of the amount of Al added is about 3% in the production method of the optical fiber amplifier, and the resulting optical fiber amplifier has a gain of 26 dB and a gain of 26 dB. A 3 dB bandwidth of about 25 nm is not sufficient. (3) The third approach is that the optical fiber length (1/2 length) is sufficiently shorter than the optimal length of the optical fiber that can achieve the maximum gain.
However, the gain is sacrificed because of the use of, and high gain and wideband characteristics cannot be satisfied. (4) The fourth and fifth attempts are not efficient because the gain is greatly suppressed in spite of the multistage configuration capable of amplifying large power. (5) The sixth attempt is excellent in that the power of each wavelength of the wavelength multiplexing signal light source is controlled while maintaining the gain band characteristic of the optical fiber amplifier, but the driving circuit of the signal light source becomes complicated. There is a problem that it is necessary to control the power of each wavelength of the signal light source according to each optical fiber amplifier.

As described above, in the conventional optical fiber amplifier (rare-earth element-doped single-core fiber amplifier), always achieving high gain and widening the band contradict each other. Could not be satisfied at the same time.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a rare earth element-doped multi-core optical fiber which satisfies a high gain and a wide band simultaneously, a method of manufacturing the same, and an optical amplifier using this optical fiber. Therefore, the first invention is a core having a high refractive index (nw) and an outer diameter (Dw) to which at least one rare earth element and Al are added, and a core which covers the core and has a low refractive index (nc). , At least three optical fiber strands formed of a cladding having a thickness (t), and a jacket for accommodating the at least three optical fiber strands, and the outer diameter (D
w) and thickness (t) (1 + 2t / Dw)
Is in the range of 1.1 to 2.5, and the additive amount of Al is at least 1% by weight.

In a second aspect, the rare earth element is Er, Y
b. Pr, Nd, Ce, Sm, Ho.
The third invention provides a rare-earth-element-doped multi-core optical fiber according to the above paragraph, wherein the relative refractive index difference Δn1 (= (nw−nc) / nw × 100) between the core and the clad is provided.
%) And the relative refractive index difference Δn2 between the core and the jacket when the refractive index of the jacket is nj.
(= (Nw-nj) / nw × 100%) is 1 to 3%
The rare earth element-doped multi-core optical fiber according to claim 1 or 2, wherein the outer peripheral length of the core accommodated in the jacket is Dg, and When the outer diameter of the jacket is Do
The rare-earth element-doped multi-core optical fiber according to any one of claims 1 to 3, wherein the value of o / Dg is in the range of 12.5 to 25. The rare-earth element-added multi-core light according to any one of claims 1 to 4, wherein the rare-earth elements added to the core are Er and Yb, and the amount of Er added is at least 200 ppm by weight. A sixth aspect of the present invention provides a fiber, wherein the core is formed of SiO 2 -based glass,
6. The method according to claim 5, wherein at least one additional substance for controlling refractive index such as Ge, P, or B is added in addition to r, Yb, and Al.
13. A rare earth element-doped multi-core optical fiber according to item 5.

According to a seventh aspect of the present invention, there is provided a core having a high refractive index (nw) and an outer diameter (Dw) to which at least one rare earth element and Al are added, and a low refractive index ( nc) and at least three optical fiber strands composed of a clad having a thickness (t),
w) and thickness (t) (1 + 2t / Dw)
Is prepared in a range of 1.1 to 2.5 and the amount of Al added satisfies the condition of at least 1% by weight, and the at least three optical fibers are inserted into a jacket tube, A method for producing a rare-earth element-doped multi-core optical fiber, characterized in that a composite is formed by welding such that no gap is formed between a fiber strand and the jacket tube, and the composite is heated at a high temperature and drawn.

In an eighth aspect, at least one rare earth element and Al are added, and a high refractive index (nw) and an outer diameter (Dw) are added.
And a core having a low refractive index (n
c) At least three optical fiber wires composed of a clad having a thickness (t) and a value (1 + 2t / Dw) determined by the outer diameter (Dw) and the thickness (t) are 1.1.
２．５2.5, wherein the amount of Al is at least 1% by weight and prepared, and the at least three optical fibers are inserted into a jacket tube, A method for manufacturing a rare-earth element-doped multi-core optical fiber, comprising: forming a composite by welding under high temperature and high heat so as to eliminate a gap between the core tube and the jacket tube; and drawing the composite.

In a ninth aspect of the present invention, the step of preparing the composite includes a step of filling a low refractive index material between the optical fiber and the jacket tube. Provided is a method for manufacturing the described rare earth element-doped multi-core optical fiber. Further, in the tenth aspect, at least one rare earth element and Al are added, and a high refractive index (nw) is provided.
And at least three optical fiber strands comprising a core having an outer diameter (Dw) and a cladding covering the core and having a low refractive index (nc) and a thickness (t). A value (1 + 2t / Dw) determined by the outer diameter (Dw) and the wall thickness (t) is in the range of 1.1 to 2.5, and the addition of the Al A rare-earth element-doped multi-core optical fiber having an amount of at least 1% by weight, a first isolator for passing an input signal light, and a signal light output from the first isolator and an excitation light from an excitation light source. A first optical demultiplexer input to an input part of a fiber, and a second optical demultiplexer for separating the pump light output from the output part of the optical fiber and the signal light amplified in the optical fiber Vessel and before Providing an optical amplifier characterized in that it is composed of a second optical isolator for passing the amplified the signal light output from the second optical demultiplexer.

According to an eleventh aspect, at least one rare earth element and Al are added, and a high refractive index (nw) and an outer diameter (D
w), and at least three optical fiber strands comprising a clad covering the core and having a low refractive index (nc) and a thickness (t);
A value (1 + 2t / Dw) determined by the outer diameter (Dw) and the thickness (t) is in the range of 1.1 to 2.5, and A rare-earth element-doped multi-core optical fiber having an addition amount of at least 1% by weight, a first isolator for passing the input signal light, and a signal light output from the first isolator being input to an input portion of the optical fiber And a first optical demultiplexer that separates the pump light output from the input unit, and separates the amplified signal light output from the output unit of the optical fiber, and outputs the pump light from the pump light source. A second optical demultiplexer input to the output unit; and a second optical isolator that passes the amplified signal light separated by the second optical demultiplexer. Provide optical amplifiers

According to a twelfth aspect, at least one rare earth element and Al are added, and a high refractive index (nw) and an outer diameter (D
w), and at least three optical fiber strands comprising a clad covering the core and having a low refractive index (nc) and a thickness (t);
A value (1 + 2t / Dw) determined by the outer diameter (Dw) and the thickness (t) is in the range of 1.1 to 2.5, and A rare-earth element-doped multi-core optical fiber having an addition amount of at least 1% by weight, a first isolator that allows input signal light to pass therethrough, a signal light output from the first isolator, and an excitation light from an excitation light source. A first optical demultiplexer that is input to an input section of the optical fiber, and separates the amplified signal light output from an output section of the optical fiber, and outputs pump light from another pump light source to the output section. An optical amplifier comprising: a second optical demultiplexer input to the second optical demultiplexer; and a second optical isolator that passes the amplified signal light separated by the second optical demultiplexer. I will provide a.

In a thirteenth aspect, an area for mode field matching is provided between the input and output sections of the optical fiber and the first and second optical demultiplexers. 13. An optical amplifier according to any one of items 10 to 12. According to the first aspect, a rare-earth element-doped multi-core optical fiber for an optical fiber amplifier that satisfies both high gain and a wide band can be obtained. This is due to the adoption of the following two means.

First, the multi-core optical fiber of the present invention is manufactured by a specific manufacturing method. That is, the rare earth element-doped multi-core optical fiber of the present invention,
A rare earth element and Al of a value close to the upper limit in advance (the maximum value that can be realized with the conventional rare earth element-doped single-core optical fiber)
Prepare at least three optical fiber strands having a high-refractive-index core containing a low-refractive-index cladding, and insert these optical fiber strands into a low-refractive-index jacket tube;
An optical fiber is manufactured by producing a hand optical fiber preform by vacuum evacuation and welding so as to eliminate the gap between the inner wall of the jacket tube and the optical fiber, and drawing the preform by heating at a high temperature. . By this manufacturing method, the total amount of the rare earth element added to the obtained optical fiber can be increased to about three times or more as compared with the conventional rare earth element-doped single core optical fiber, so that a high gain is achieved. . Further, since the total amount of Al added can be increased to about three times or more as compared with the conventional case, the band can be significantly widened.

Second, the optical fiber of the present invention has a specific structure. That is, the optical fiber of the present invention has a structure in which the value of the relational expression (1 + 2t / Dw) between the thickness t of the first cladding and the core outer diameter Dw is in the range of 1.1 to 2.5. Thus, it is possible to propagate at approximately the same binding efficiency LP ₀₁ mode of the excitation light and signal light in each core enters the rare-earth-doped core optical fiber (≧ 80%). Therefore, the incident pump light is substantially equally distributed into each core, and the signal light is also substantially equally distributed. Therefore, the pumping light power becomes 1 / n (n: the number of cores) as compared with the conventional rare earth element-doped single core optical fiber, and the amplification degree of the signal light propagating in each core, that is, the gain decreases. However, a uniform amplification degree (gain) can be obtained over a wide signal light wavelength band.

By the above two means, the rare earth element-doped multi-core optical fiber of the present invention can satisfy both high gain and wide band at the same time. The reason why the value of the above relational expression (1 + 2t / Dw) should be selected in the range of 1.1 to 2.5 is that if the value is smaller than 1.1, a structure in which the cores are almost in contact with each other (conventionally, The structure is close to a rare-earth element-doped single-core optical fiber), and it is difficult to broaden the band. On the other hand, if this value is larger than 2.5, a part of the power of the signal light and the pumping light will be reduced in the first clad other than the core. In addition, the signal propagates through the jacket and does not contribute to amplification, so that a high gain cannot be achieved after all.

According to the second aspect of the present invention, E is used as a rare earth element.
r, Yb, Pr, Nd, Ce, Sm, Ho and the like can be used. That is, the present invention is also applicable to optical fiber amplifiers of various wavelength bands. As a specific example, Er or Er
In the case of an optical fiber doped with Yb and Yb, the wavelength 1.
It is possible to amplify the signal light over the range of 53 to 1.57 μm. In the case of an optical fiber doped with Pr or Nd, signal light in the 1.3 μm band can be amplified.

According to the third aspect of the present invention, by setting the relative refractive index differences Δn1 and Δn2 in the range of 1 to 3%, the confinement rates of the signal light and the pumping light in each core are increased, and the Gain and broadband can be further promoted. In addition,
Although it is desirable that Δn1 and Δn2 be 3% or more, it is difficult in terms of manufacturing technology at present. Further, ordinary low-mode single-mode fibers are connected to the input and output ends of the optical fiber of the present invention, and these connection portions are provided with mode field matching.

According to the fourth aspect, Do / Dg is set to 12.
By setting it in the range of 5 to 30, the same effect as that of the first invention can be expected, and at the same time, a single-mode fiber with a normal low △ (differential refractive index difference) connected to the input / output end of the optical fiber can be obtained. Low loss mode field matching connection can be realized. On the other hand, Do / Dg is 1
If it is smaller than 2.5 or larger than 30, mode field matching with a normal low △ (△: ０．３0.3%) single mode fiber becomes difficult to obtain, resulting in a decrease in gain. .

According to the fifth and sixth aspects of the present invention, as a material of each core, a material obtained by adding Er and Yb to SiO2-GeO2-Al2O3-based glass is used. By setting the content to at least 200 ppm by weight, higher gain can be achieved while maintaining the broadband characteristics. The signal light in this case uses a 1.5 μm band, and the wavelength of the pump light is preferably 1.053 μm.
It may be 0.98 μm or 1.48 μm. The material of each core is made of SiO 2 -based glass to which Er, Yo, and Al are added. In order to make the relative refractive index differences Δn 1 and Δn 2 1 to 3%, at least Ge, P, B, etc. It is preferable to add one type. The cladding and the jacket are also made of SiO2 material.
When an additive such as F that lowers the refractive index is used, larger Δn1 and Δn2 can be obtained. In this case, n
The values of c and nj may be equal or different.

According to the seventh and eighth aspects, a rare-earth element-doped multi-core optical fiber having desired structural parameters can be manufactured with good reproducibility. That is, since the optical preform is manufactured by welding a plurality of optical fiber strands in which the structural parameters are controlled in advance in the jacket tube, the structural parameters can be managed for each process. As a result, an optical fiber as designed can be manufactured with good reproducibility.

According to the ninth aspect, the shape of each core in the cross section of the manufactured optical fiber can be kept substantially circular, and the value of (1 + 2t / Dw) can be easily adjusted. Further, an optical fiber with little polarization dependence can be manufactured. According to the tenth and eleventh aspects, it is possible to realize an optical amplifier that satisfies both high gain and wide band.

According to the twelfth aspect, an optical amplifier having a higher gain and a wider band can be realized. The use of the optical amplifier according to the tenth to twelfth aspects enables transmission over a longer distance than conventional optical amplifiers and achieves multi-channel wavelength division multiplexing transmission. Therefore, a very economical system can be constructed. Further, since there is no need to use extra optical components, cost reduction and size reduction can be achieved. Further, since sufficient optical characteristics can be obtained with one amplifier, the noise figure of the transmission system can be minimized, and high-quality information transmission can be achieved.

According to the thirteenth aspect, generation of unnecessary reflected light in the optical amplifier can be suppressed, and energy conversion loss can be reduced. Therefore,
A stable optical amplification system can be constructed.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a rare earth element-doped multi-core optical fiber according to the present invention will be described below. The embodiment shown in FIG. 1 includes seven cores (designated 5-1 to 5-7, respectively). The cores 5-1 to 5-7 are made of Ge on SiO2.
O2, Al2 O3 and Er ions are added, and 1,000 ppm by weight of Al and 300 ppm by weight of Er are added. A clad 6 having a thickness t is formed on the outer periphery of each of the cores 5-1 to 5-7. And this core 5
-1 to 5-7 are covered with a jacket 7. The outer diameter Do of the jacket 7 is 125 μm, the outer diameter Dg of the seven assembled cores is 4.4 μm (Do / Dg = 28.4),
The outer diameter Dw of each core is 1.23 μm, (1 + 2t / Dw)
Is about 1.2, the core refractive index nw is 1.48, the cladding refractive indexes nc and nj are 1.458, and the relative refractive index △ n
1 and Δn2 are the case of 1.509%.

The sectional view of the optical fiber shown in FIG. 1 is a structural view in an ideal case.
1 to 5-7 are put in a jacket glass tube (in this case, a quartz glass tube), and the claddings 6 are welded to each other and the inner wall of the glass tube while vacuum is applied. An optical fiber is obtained by drawing as follows. Therefore, the cross section of each core of the obtained optical fiber is deformed from a circle as shown in FIG.

In the optical fiber shown in FIG.
A pump light having a wavelength of 1.55 μm and a signal light having a wavelength of 1.55 μm were made incident, and the length of the optical fiber at which the gain was almost maximized was obtained.
2 m. When the power of the signal light was -27 dBm and the power of the pump light was 40 mW, the gain was about 25 dB and the noise figure was 5 dB. In addition, the relationship between the output power of the amplified signal light, the gain, and the noise figure had characteristics as shown in FIG. Next, the wavelength of the signal light is set to 1.53 to 1.56 μm.
The relationship between the gain and the noise figure when changing to m was as shown in FIG. That is, the gain is maximum (25d
The 3 dB bandwidth in the case of B) and the case of 21 dB were both 30 nm or more. In particular, the power of the signal light is -27.
It can be seen that when the gain is increased from dBm to -17 dBm and the gain is suppressed from about 25 dB to 21 dB, the gain variation is suppressed to ± 1 dB or less over a wide bandwidth of 30 nm or more. It is a feature of the rare-earth element-doped multi-core optical fiber of the present invention that such a broadband characteristic can be realized.

As a second embodiment, in FIG. 1, the outer diameter Do of the jacket 7, the refractive indices nw, nc, nj of the core, the clad and the jacket, and the values of the relative refractive index differences Δn1 and Δn2 are not changed. Outer diameter Dg of 4.4 μm
To 6.5 μm (Do / Dg = 19.2), and the outer diameter Dw of each core from 1.23 μm to 1.81 μm,
An optical fiber in which (1 + 2t / Dw) was changed from about 1.20 to about 1.36 was manufactured, and the optical characteristics were evaluated in the same manner as in the first example. As a result, the pumping light having a wavelength of 1.48 μm and the signal light having a wavelength of 1.55 μm were made incident, and the optical fiber length at which the gain was almost maximized was found to be about 25 m.
The power of the signal light is -27 dBm, and the power of the pump light is 40
The gain at mW was about 32 dB, which was higher than that of the first embodiment. This is because the value of (1 + 2t / Dw) was increased from about 1.20 to about 1.36.

FIG. 5 shows the relationship between the signal light power, the gain, and the output power of the amplified signal light. The 3 dB bandwidth of this optical fiber was about 30 nm with a gain of 31 dB.
As a third embodiment, the outer diameter Do of the jacket 7 in FIG.
The values of c, nj and the relative refractive indices Δn1 and Δn2 are kept as they are, and the outer diameter Dg of the multi-core is changed from 4.4 μm to 9.72.
μm (Do / Dg = 12.9), and the outer diameter Dw of each core.
From 1.23 μm to 1.95 μm, and (1 + 2t
/ Dw) was changed from about 1.20 to about 2.29, and the optical characteristics were evaluated in the same manner as in the first embodiment. As a result of evaluation using a light source having a wavelength of 0.98 μm as the excitation light, the optical fiber length at which the gain is almost maximum is about 32.
m. The gain was about 37 dB when the power of the signal light was -37 dBm and the power of the pump light was 120 mW. Also, as a result of measuring the bandwidth, as shown in FIG.
Broadband characteristics could be obtained with high gain. That is, when the signal light power is −7 dBm and −17 dBm, the characteristics are almost flat from the wavelength of 1.53 μm to 1.56 μm, and the gains are 22 dB and 31 dB. The 3dB bandwidth is about 30n even when the gain is 35dB.
m, which is a wide band.

As described above, it has been found that by increasing (1 + 2t / Dw), a wide band characteristic can be realized with a high gain. This is presumed to be due to the fact that the pump light and the signal light are almost equally distributed to each core. As a result, the gain of the signal light in each core is low, but the broadband characteristic is correspondingly obtained. Then, the gain of the signal light in each core is added to the power on the output side of the optical fiber, and as a result, the gain becomes high while maintaining the wideband characteristic. To increase the bandwidth and increase the gain, in principle, it is better to increase the number of cores. However, if the number of cores is too large, the distance between the cores becomes very narrow, so that the optical fiber approaches the appearance of a single-core optical fiber. .
Therefore, it is necessary that (1 + 2t / Dw) be in the range of 1.1 to 2.2 and Do / Dg be in the range of 12.5 to 30.

As a fourth embodiment, the amount of Al added to each of the cores 5-1 to 5-7 in the second embodiment is set to 1, 0
From 00 ppm by weight to 4,000 ppm by weight,
An optical fiber was manufactured in which r was increased from 300 ppm by weight to 400 ppm by weight. As a result, the relationship between the signal light power, the gain, and the output power of the amplified signal light is shown in FIG. Here, Pp is the power of the excitation light having a wavelength of 1.48 μm. The maximum gain was 42 dB. The wavelength characteristics of the gain measured by changing the wavelength of the signal light are shown in FIG. 8, and the maximum gain was 42 dB and the 3 dB bandwidth was 30 nm. The band characteristics at gains of 36 dB and 31 dB are ± 0.6 dB over a wavelength range of 1.53 μm to 1.56 μm.
It has been found that a flat gain characteristic can be obtained over an extremely wide band having a gain deviation of B or less. From this result, it is clear that a further flat gain band characteristic can be realized by further increasing the amount of Al added to each core. Further, a higher gain can be realized by further increasing the amount of Er added. Further gain enhancement can be achieved by adding Yb as a rare earth element other than Er in each core and exciting the core with excitation light having a wavelength of 1.053 μm.

In the above embodiment, Er is used as the rare earth element. However, a multi-core optical fiber using other rare earth elements can also obtain the characteristics satisfying both the high gain and the wide band as in the above embodiment. It is possible. For example, Pr
Can be used to amplify the signal light in the 1.3 μm band, and when Nd is used, the signal light in the 1.3 μm band or 1.06 μm band can be amplified.

As a fifth embodiment, the refractive index nw of the core is changed from 1.48 to 1.479 in the second embodiment.
The relative refractive indices Δn1 and Δn2 are changed from 1.509% to 1.4.
Changing to 2% resulted in a slight decrease in gain (0.4 dB). Conversely, the core refractive index nw is changed from 1.48 to 1.485, and the relative refractive indices Δn1 and Δn2 are set to 1.48.
As a result of the increase to 8%, the gain increased by about 1.5 dB.

The number of cores shown in FIG. 1 may be at least three, but is preferably 7, 10, 14, 19,.
・ Can be as many as Next, an embodiment of the optical amplifier using the rare earth element-doped multi-core optical fiber of the present invention will be described. FIG. 9 shows a first embodiment of the optical amplifier according to the present invention, which includes an optical isolator 10-1, an optical demultiplexer 11-.
1. Rare earth element-doped multi-core optical fiber 8, optical demultiplexer 11-2, optical isolator 10-2, pump light source 13, and two mode field matching regions 14-1 and 1
4-2.

The signal light 9-1 is input to the optical isolator 10-1, and the pump light 12-1 from the pump light source 13 is supplied to the rare-earth element-doped multi-core optical fiber 8 through the optical demultiplexer 11-1 and amplified. The remaining pump light that has not contributed to is extracted as indicated by the arrow 12-2 via the optical demultiplexer 11-2. The optical isolator 10-1 suppresses the signal light amplified by the optical fiber 8 from being reflected on the way and returned to the input side. In addition, the other optical isolator 10-2 outputs the amplified signal light 9-.
This is for suppressing the light 2 from being reflected on the way and returning to the optical fiber side.

Mode field matching areas 14-1 and 14
-2 is the relative refractive index difference Δn1 and Δn2 of the optical fiber 8
Is in the range of 1 to 3%, whereas the optical demultiplexer 11-
Since the relative refractive index difference of the optical fiber transmission line on the 1 and 11-2 side where no rare earth element is added is about 0.3%,
It is provided to suppress reflected waves and reduce reflection loss due to mode mismatching due to the difference in relative refractive index.

The configuration shown in FIG. 9 is an embodiment of an optical fiber amplifier using forward pumping. When Er or Er and Yb are added as the rare earth element of the rare earth element-doped multi-core optical fiber 8, the excitation light source 13
Has a wavelength of 0.9 μm, 1.053 μm or 1.48 μm.
m light sources are used. FIG. 10 shows another embodiment of a multi-core optical fiber amplifier doped with a rare earth element by backward pumping.
The pumping light source 13 is provided on the side of the optical demultiplexer 11-2.
-2. Other configurations are the same as those in FIG.

FIG. 11 shows another embodiment of the rare earth element-doped multi-core optical fiber amplifier in the case of forward and backward pumping. In this case, the excitation light sources are 13-1 and 1
Two are provided as shown in 3-2. By performing bidirectional pumping in this way, higher gain amplification can be realized. In this case, the wavelength of the excitation light sources 13-1 and 13-2 can be selected from 0.98 μm, 1.053 μm, and 1.48 μm. That is, the wavelengths having the same wavelength or different wavelengths may be used.

FIG. 12 shows an embodiment of a method for manufacturing a rare earth element-doped multi-core optical fiber according to the present invention. First, (a)
As shown in (1), a high refractive index core rod containing a rare earth element and Al is prepared, and a low refractive index clad is formed around the core rod. This cladding is formed by using a usual method for manufacturing an optical fiber preform. For example, flame deposition,
An external CVD method or the like is used. Next, as shown in (b), a plurality of optical fibers are inserted into a jacket tube having a low refractive index. Here, a quartz tube is used as the jacket tube,
A tube having a low refractive index film formed on the inner wall surface of a quartz tube, a Vycor glass tube (trade name of Corning Glass Co., Ltd.), or the like can be used. Thereafter, as shown in (c), the gap between the inner wall of the jacket tube and the optical fiber is welded while being heated at a high temperature from the outer periphery of the jacket tube to obtain a rod-shaped optical fiber preform. At the time of the above welding, one end of the jacket tube is sealed, and the jacket is heated at a high temperature while being evacuated from the other end and welded to obtain an optical fiber preform with less residual bubbles. Finally, as shown in (d), the optical fiber preform is drawn into the optical fiber while being heated using a usual optical fiber drawing method.

In the step (c) of the above manufacturing method, a material having a low refractive index (for example, SiO2 rod, SiO2 fine powder, SiO2 containing B2O3 or F2) is formed in the gap between the inner wall of the jacket tube and the optical fiber during welding. Rod or fine powder, etc.).

[0044]

As described above in detail, the following effects can be obtained by the rare earth element-doped multi-core optical fiber of the present invention, the method of manufacturing the same, and the optical amplifier using this optical fiber. (1) Both high gain and wide band can be satisfied. (2) Even for optical fiber amplifiers in various bands, characteristics satisfying both high gain and wide band can be obtained. (3) By adding Er and Yb as rare earth elements, it is possible to realize a higher gain while maintaining a wide band characteristic. (4) With the optical amplifier of the present invention, further long-distance transmission and multi-channel wavelength multiplex transmission can be realized. (5) A more economical system can be constructed. (6) The noise figure of the optical amplifier system can be minimized, which enables high-quality information transmission. (7) A rare earth-doped multi-core optical fiber having desired structural parameters can be produced with good reproducibility.

[Brief description of the drawings]

FIG. 1 shows a cross-sectional view of an embodiment of a rare earth element-doped multi-core optical fiber of the present invention.

FIG. 2 shows a cross-sectional view of an embodiment of the rare earth element-doped multi-core optical fiber of the present invention.

FIG. 3 is a graph showing the optical characteristics of the rare earth element-doped multi-core optical fiber of the present invention.

FIG. 4 is a graph showing optical characteristics of the rare-earth element-doped multi-core optical fiber of the present invention.

FIG. 5 is a graph showing optical characteristics of the rare-earth element-doped multi-core optical fiber of the present invention.

FIG. 6 is a graph showing the optical characteristics of the rare earth element-doped multi-core optical fiber of the present invention.

FIG. 7 is a graph showing the optical characteristics of the rare earth element-doped multi-core optical fiber of the present invention.

FIG. 8 is a graph showing the optical characteristics of the rare earth element-doped multi-core optical fiber of the present invention.

FIG. 9 shows an embodiment of the optical amplifier of the present invention.

FIG. 10 shows an embodiment of the optical amplifier of the present invention.

FIG. 11 shows an embodiment of the optical amplifier of the present invention.

FIG. 12 shows a method for manufacturing a rare earth element multi-core optical fiber of the present invention.

FIG. 13 is a structural view of a rare-earth element-doped multi-core fiber previously proposed by the present inventors.

[Explanation of symbols]

 5-1 to 5-7 Core 6 Clad 7 Jacket 8 Rare earth element doped multi-core optical fiber 9-1, 9-2 Signal light 10-1, 10-2 Optical isolator 11-1, 11-2 Optical splitter 12- 1, 12-2 Excitation light 13 Excitation light source 14-1, 14-2 Mode field matching region

────────────────────────────────────────────────── (5) Int.Cl. ⁶ identification symbol FI H01S 3/10 H01S 3/10 Z (56) References JP-A-5-299733 (JP, A) JP-A-5-345632 ( JP, A) JP-A-6-216441 (JP, A) JP-A-6-216440 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 6/04 G02B 6/16 H01S 3/07 H01S 3/10

Claims (13)

(57) [Claims] A core having a high refractive index (nw) and an outer diameter (Dw) added with at least one rare earth element and Al;
And at least three optical fiber strands which cover the core and are composed of a clad having a low refractive index (nc) and a thickness (t); and a jacket accommodating the at least three optical fiber strands. A value (1+) determined by the outer diameter (Dw) and the wall thickness (t).
2t / Dw) is in the range of 1.1 to 2.5, and the Al
A rare earth element-doped multi-core optical fiber, characterized in that the amount of addition is at least 1% by weight. 2. The method according to claim 1, wherein the rare earth element is Er, Yb, Pr, N
The rare earth element-doped multi-core optical fiber according to claim 1, wherein the optical fiber is d, Ce, Sm, or Ho. 3. The method according to claim 1, wherein a relative refractive index difference Δn1 (= (nw−nc) / nw × 100%) between the core and the clad, and the refractive index of the jacket is nj. The relative refractive index difference Δn2 (= (n
The rare-earth element-doped multi-core optical fiber according to claim 1 or 2, wherein both (w-nj) / nw x 100%) are in the range of 1 to 3%. 4. The value of Do / Dg is in the range of 12.5 to 25, where Dg is the outer peripheral length of the core accommodated in the jacket, and Do is the outer diameter of the jacket.
The rare-earth element-doped multi-core optical fiber according to any one of claims 1 to 3. 5. The rare earth element added to the core is Er and Yb, and the amount of Er added is at least 2%.
The rare-earth element-doped multi-core optical fiber according to any one of claims 1 to 4, wherein the amount is 00 ppm by weight. 6. The method according to claim 5, wherein said core is made of SiO 2 glass, and at least one additional material for controlling the refractive index such as Ge, P and B is added in addition to Er, Yb and Al. The rare-earth element-doped multi-core optical fiber described in the above. 7. A core to which at least one rare earth element and Al are added and which has a high refractive index (nw) and an outer diameter (Dw).
And at least three optical fiber strands covering the core and composed of a clad having a low refractive index (nc) and a thickness (t) are determined by the outer diameter (Dw) and the thickness (t). Value (1+
2t / Dw) is in the range of 1.1 to 2.5, and the Al
Is prepared so as to satisfy the condition of at least 1% by weight, and the at least three optical fiber wires are inserted into a jacket tube so that no gap is formed between the optical fiber wire and the jacket tube. A method for producing a rare-earth-element-doped multi-core optical fiber, comprising: welding to form a composite; and drawing the composite by heating at a high temperature. 8. A core having a high refractive index (nw) and an outer diameter (Dw) added with at least one rare earth element and Al,
And at least three optical fiber strands covering the core and composed of a clad having a low refractive index (nc) and a thickness (t) are determined by the outer diameter (Dw) and the thickness (t). A value (1 + 2t / Dw) is in the range of 1.1 to 2.5, and the amount of Al added is prepared so as to satisfy the condition of at least 1% by weight, and the at least three optical fibers are jacketed. A rare earth element-added multi-core, which is inserted into a tube and welded under high temperature and high heat to form a composite so that no gap is formed between the optical fiber and the jacket tube, and then the composite is drawn. Optical fiber manufacturing method. 9. The rare earth element according to claim 7, wherein the step of preparing the composite includes a step of filling a low refractive index material between the optical fiber and the jacket tube. Manufacturing method of doped multi-core optical fiber. 10. A core having a high refractive index (nw) and an outer diameter (Dw) to which at least one rare earth element and Al are added, and a low refractive index (nc) and a thickness ( At least three optical fiber strands composed of a clad having t) and a jacket accommodating the at least three optical fiber strands, and are determined by the outer diameter (Dw) and the wall thickness (t). Value (1+
2t / Dw) is in the range of 1.1 to 2.5, and the Al
A multi-core optical fiber doped with at least 1% by weight of a rare earth element, a first isolator for passing an input signal light, and a signal light output from the first isolator and an excitation light from an excitation light source. A first optical demultiplexer to be input to an input section of the optical fiber; and a second optical splitter for separating the pump light output from the output section of the optical fiber and the signal light amplified in the optical fiber.
An optical amplifier, comprising: a second optical isolator that allows the amplified signal light output from the second optical demultiplexer to pass therethrough. 11. A core having a high refractive index (nw) and an outer diameter (Dw) to which at least one rare earth element and Al are added, and a core having a low refractive index (nc) and a thickness ( At least three optical fiber strands composed of a clad having t) and a jacket accommodating the at least three optical fiber strands, and are determined by the outer diameter (Dw) and the wall thickness (t). Value (1+
2t / Dw) is in the range of 1.1 to 2.5, and the Al
Rare earth element-doped multi-core optical fiber having an addition amount of at least 1% by weight, a first isolator that allows input signal light to pass through, and a signal light output from the first isolator is input to an input portion of the optical fiber. A first optical demultiplexer that inputs and separates pump light output from the input unit, and separates the amplified signal light output from an output unit of the optical fiber, and pump light from a pump light source. And a second optical isolator that passes the amplified signal light separated by the second optical demultiplexer. Optical amplifier. 12. A core having a high refractive index (nw) and an outer diameter (Dw) to which at least one rare earth element and Al are added, and a core having a low refractive index (nc) and a thickness ( At least three optical fiber strands composed of a clad having t) and a jacket accommodating the at least three optical fiber strands, and are determined by the outer diameter (Dw) and the wall thickness (t). Value (1+
2t / Dw) is in the range of 1.1 to 2.5, and the Al
Rare-earth element-doped multi-core optical fiber having an addition amount of at least 1% by weight, a first isolator that allows input signal light to pass therethrough, a signal light output from the first isolator, and an excitation light from an excitation light source A first optical demultiplexer for inputting the signal light to an input portion of the optical fiber, separating the amplified signal light output from an output portion of the optical fiber, and outputting the pump light from another pump light source to the output portion. A second optical demultiplexer input to the unit, and a second optical isolator that passes the amplified signal light separated by the second optical demultiplexer. amplifier. 13. The apparatus according to claim 10, wherein an area for mode field matching is provided between said input and output sections of said optical fiber and said first and second optical demultiplexers. Item 13. The optical amplifier according to Item 12.